Method of making a semiconductor light emitting device using out-diffusion from a buried stripe

ABSTRACT

A buried stripe semiconductor light emitting device and a method for producing the device in which the buried stripe functions as an internal resonator, and the device has window regions interposed between the resonator and facets on the external surface of the device. A first phase crystal growth is conducted in which a first cladding layer is grown on a doped substrate. Thereafter, a doped stripe of impurities is introduced into the first cladding layer in electrical contact with the doped substrate. The doped stripe extends longitudinally but terminates short of the facets so that later out-diffusion from the doped stripe will form the window regions. A second phase crystal growth is then conducted which buries the doped stripe internal to the semiconductor, i.e., not projecting through any external surface. The second phase crystal growth comprises an active layer, a second cladding layer and a contact layer successively grown on the first cladding layer. Impurities from the buried doped stripe are out-diffused into the active layer to the boundary between the active layer and the second cladding layer to form the resonator, leaving windows interposed between the resonator ends and the facets.

This is a Division, of application Ser. No. 216,832, filed July 8, 1988,U.S. Pat. No. 4,888,782.

FIELD OF THE INVENTION

This invention relates to semiconductor light emitting devices. Moreparticularly, the invention relates to a window structure semiconductorlight emitting device having a high power output, and a method forproducing such device.

BACKGROUND OF THE INVENTION

A prior art window structure semiconductor laser is described in "AnAlGaAs Window Structure Laser", IEEE Journal of Quantum Electronics,Vol. QE-15, No. 8, August, 1979, pp. 775-781. FIG. 3(a) is a plan viewof such a device, while FIGS. 3(b) and 3(c) are cross-sectional viewstaken along lines 3b--3b and 3c--3c of FIG. 3(a), respectively.

As illustrated in FIGS. 3(a)-3(c), a semiconductor light emitting deviceusing the growth techniques of the prior art starts with an n-type GaAssubstrate 1. A first cladding layer 2, of n-type Al_(y) Ga₁₋₇ As isgrown on the n-type GaAs substrate 1. Thereafter, an active layer 3 isgrown on the first cladding layer 2 through conventional growthtechniques. As is understood in the art, active layer 3 is either ofn-type or undoped Al_(x) Ga_(1-xl) As (x<y). Similar to the firstcladding layer 2, a second cladding layer 4 of the same n-type Al_(y)Ga_(1-y) As the first cladding layer 2 is next grown on the active layer3. The cladding layers 2, 4 have a lower refractive index than theactive layer 2, thus confining light generated in the active layer towithin that layer. Contact layer 5 is then grown on cladding layer 4 andis of n-type GaAs, completing the growth process of a prior artsemiconductor light emitting device. Although not illustrated in FIG. 3,an n side electrode is formed on the substrate 1 and a p side electrodeon the contact layer 5 for making electrical connections with thesemiconductor device.

In order to create a p-n junction for injecting carriers into apredetermined section of the active layer 3, a first p-type impurityregion 6 is diffused from the surface of the contact layer 5 through thesecond cladding layer 4 to approximately the junction between the secondcladding layer 4 and the active layer 3. Thereafter, thermal processingproduces a second p-type diffusion region 7 by out-diffusing impuritiesfrom the first p-type diffusion region 6 into the active layer 3 toapproximately the boundary between the active layer 3 and first claddinglayer 2. Thus, there is created a stripe region 8 in the active layerwhich (a) forms a p-n junction with the adjacent portion of the firstcladding layer, and (b) creates a resonator which has a higher index ofrefraction than the cladding layers or the undoped or n-doped regions ofthe active layer to confine light within the resonator.

As best shown in FIGS. 3(a) and 3(c), the first and second diffusionregions 6, 7 are positioned longitudinally of the laser, but terminateshort of the facets 10, 10' which are located on either longitudinal endsurface of the laser. Thus, a pair of windows 9, 9' are created betweenthe stripe region or resonator 8 and the facets 10, 10'. By virtue ofthe impurity concentration in the resonator 8 (which it is recalled iscreated by out-diffusion from the stripe region 7), the energy band gapof the resonator 8 is smaller than that of the undoped or n-type dopedregion of the active layer 3 at the windows 9, 9'. By virtue of thisdifference in energy gaps, very little of the light generated in theresonator 8 is absorbed in the windows 9, 9', allowing the laser to beoperated at higher power.

Although this prior art light emitting device solves the problem ofdevice failure at high operating power by providing windows interposedbetween the resonator and the laser facets, in contrast to other priordevices where the resonator intersects the facets, it appears to sufferfrom the problem of inadequate yield because of manufacturingdifficulties. More particularly, the location of the out-diffusionregion 7, particularly the portion which forms the resonator 8 must becarefully controlled in order to produce useful laser devices.Preferably, the resonator portion 8 of the out-diffusion region 7penetrates the active layer 3 and terminates at the boundary between theactive layer and first cladding layer 2. However, because of thedifficulty of controlling the diffusion depth of the first diffusionregion 6, and consequently the difficulty of controlling the location ofthe significantly smaller out-diffusion region 7, it often happens thatthe resonator portion 8 of the out-diffusion region 7 does not reach theactive layer, or alternatively it penetrates the active layer andprotrudes into the first cladding layer 2. Both of these situationsproduce laser devices which are non-functional, thus reducing the yieldof the process. When it is appreciated that the cladding layers are onthe order of 2 or more microns whereas the active layer is typicallyabout 0.1 microns, the difficulty of process control will be apparent,first of all, in controlling the depth of the first diffusion region toprovide an appropriate starting point for the out-diffusion, andsubsequently in controlling the out-diffusion to form the resonator.This problem can be further aggravated by surface irregularities on thegrown crystal which can be exacerbated by the multiple levels which aregrown before the first diffusion region is formed.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide a window structure semiconductor light emitting device capableof being produced at a higher yield than has previously been possible.

More specifically, it is an object of the present invention to provide amethod of producing a semiconductor light emitting device such thatgreater control over the out-diffusion region is possible.

Additionally, it is an object of the present invention to provide alight emitting device and method of producing such a device which isunaffected by variations in thickness of the active layer, secondcladding layer and contact layer.

Other objects and advantages of the present invention will becomeapparent from the detailed description given hereinafter. It should beunderstood, however, that the detailed description and specificembodiments are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention asdefined by the appended claims will become apparent to those skilled inthe art from this detailed description.

The foregoing objects are accomplished by the present invention whichrelates to a buried stripe semiconductor light emitting device and amethod for producing the device, in which the buried stripe functions asan internal resonator, and the device has window regions interposedbetween the resonator and facets on the external surface of the device.A first phase crystal growth is conducted in which a first claddinglayer is grown on a doped substrate. Thereafter, a doped stripe ofimpurities is introduced into the first cladding layer in electricalcontact with the doped substrate. The doped stripe extendslongitudinally but terminates short of the facets so that laterout-diffusion from the doped stripe will form the window regions. Asecond phase crystal growth is then conducted which buries the dopedstripe internal to the semiconductor, i.e., not projecting through anyexternal surface. The second phase crystal growth comprises an activelayer, a second cladding layer and a contact layer successively grown onthe first cladding layer. Impurities from the buried doped stripe areout-diffused into the active layer to the boundary between the activelayer and the second cladding layer to form the resonator, leavingwindows interposed between the resonator ends and the facets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a), 1(b) and 1(c) are diagrams showing a plan view andcross-sectional views of a semiconductor light emitting device accordingto the present invention;

FIGS. 2(a) and 2(b) are perspective views illustrating phases of aprocess for producing a window structure buried stripe semiconductorlight emitting device according to the present invention; and

FIGS. 3(a), 3(b) and 3(c) are a plan view and cross-sectional viewsshowing a semiconductor light emitting device according to the priorart.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, FIGS. 1 and 2 illustrate a semiconductorlight emitting device exemplifying the present invention and the methodby which it is manufactured. In contrast to the prior art where all ofthe cladding, active and contact layers are grown on the substrate in asingle crystal growth operation, in accordance with the presentinvention the crystal growth is divided into a first and second phase,with an operation intermediate the two phases which deposits a dopedstripe of impurities in close proximity to the location to be occupiedby the active layer, such that out-diffusion of impurities from thedoped stripe can be accurately controlled.

Referring more particularly to FIG. 2(a), there is shown a substrate 11,in the illustrated embodiment p-type GaAs. A first cladding layer 12comprising n-type Al_(y) Ga_(1-y) As is grown on the substrate 11 in afirst phase growth process, preferably by liquid phase epitaxy.

In contrast with conventional techniques where an active layer wouldthen be grown on the first cladding layer, in accordance with theinvention, the first phase growth process is terminated and a dopedstripe of impurities 16 is introduced into the first cladding layer. Asshown in FIGS. 2(a) and 2(b), the doped stripe 16 is internal to thesemiconductor in that when the additional layers are grown on thesemiconductor the doped stripe does not project through any externalsurface of the semiconductor. Importantly, the doped stripe 16, whichruns longitudinally of the semiconductor toward opposed facets 20, 20'has gaps 21, 21' interposed between the ends of the buried stripe 16 andthe facets 20, 20'.

The doped stripe 16 is preferably introduced by diffusion techniques,such as the closed tube diffusion process. A suitable mask, such as Si₃Ni is disposed on the first cladding layer 12 to cover all but theinternal stripe region 16, and impurities are diffused into the striperegion 16. In one typical embodiment, the closed tube diffusion processis run with a reduced pressure of about 10⁻⁷ torr. Using Zn as thesource of p-type impurities and a source of As to prevent out-diffusionfrom the GaAs, diffusion is conducted for a period of approximately 6 to7 hours. The diffusion should be conducted until the impuritiespenetrate the first cladding layer to form an electrical contact withthe p-type substrate.

It will also be clear that other techniques can be used to form theinternal doped stripe 16. For example, the stripe can be formed usingconventional ion implantation techniques in place of the diffusionprocess described above.

In practicing the invention, having disposed a source of dopedimpurities in the first cladding layer, a second phase crystal growthprocess is conducted, thereby buying the doped stripe 16, i.e., thedoped stripe 16 is completely confined within the semiconductor device,and does not project through any surface thereof. The second phasecrystal growth, preferably liquid phase epitaxy, begins with the growthof the active layer over the first cladding layer, including growing ofthe active layer over the doped stripe. The active layer is preferablyn-type Al_(x) Ga_(1-x) As (x<y), but can be undoped Al_(x) Ga_(1-x) As.In contrast to the cladding layer which can be on the order of about 2microns, the active layer is thin, preferably on the order of about 0.1microns. Following growth of the active layer, a second cladding layer14 is grown, similar in thickness to the first cladding layer, andcomprising n-type Al_(y) Ga_(1-y) As. Finally, the second phase crystalgrowth is completed by growing a contact layer 15 comprising n-type GaAsis grown on the second cladding layer 14. Although not illustrated inthe drawings, a p-side electrode is disposed on the substrate 11 and ann-side electrode on the contact layer 15 for forming electricalconnections with the semiconductor device.

In practicing the invention, some of the impurities which are originallydeposited in the dope stripe 16 are out-diffused as illustrated at 17 inthe drawings (FIGS. 1(b) and 2(b)). As best seen in FIG. 1(b), theout-diffused region 17 includes a buried stripe 18 which is out-diffusedinto the adjacent active layer 13. Diffusion conditions are controlledsuch that the out-diffused portion 18 penetrates the active layer andterminates approximately at the boundary between the active layer 13 andthe second cladding layer 14. When it is appreciated that the activelayer is only about 0.1 microns in thickness, and that the firstdiffusion region 16 is positioned immediately adjacent the active layer,the control in the location of the second diffusion region which isachieved by the invention will become apparent.

In the preferred practice of the invention, the out-diffusion region 17is created during the second phase crystal growth process. In order toaccomplish that, the time and temperature of the growth conditions areregulated to match the mobility of the diffused impurities, such thatonce the active layer 13 is deposited at the beginning of the secondphase crystal growth, during the remainder of the second phase crystalgrowth, out-diffusion of impurities from the first stripe 16 will justreach the boundary between the active layer 13 and the second claddinglayer 14 at the termination of the second phase crystal growth. Thus, inthe preferred practice of the invention, no further annealing step isrequired, and the wafer can be cleaved at its facets to form individualsemiconductor lasers immediately after the n and p electrodes aredisposed on the semiconductor as described above.

In a particular embodiment of the invention, where the impurity densitywithin the buried stripe 16 is on the order of 10²⁰ cm⁻³, the secondphase crystal growth process is conducted at an average temperature ofabout 700° C. for approximately 30 minutes in order to out-diffuse theburied stripe 18 to approximately the boundary between the active layer13 and the second cladding layer 14. The out-diffused stripe in thisembodiment can have an impurity concentration of about 10¹⁸ cm⁻³. It iswell known that crystal growth techniques include a sharp increase intemperature of the apparatus to a temperature somewhat in excess of theaverage growing temperature, a stabilization period during which thewafer stabilizes in temperature, then a growth process during which thetemperature is slowly decreased for a period of time during which theadditional layers are grown. The 30 minutes specified herein isapproximately the time from the completion of the growth of the activelayer to the termination of crystal growth following the growth of thecontact layer 15. It will be apparent that the times, temperatures andimpurity concentrations can be varied within limits by those skilled inthis art in order to achieve the specified result of growing the striperegion through the active layer to the boundary between the active andsecond cladding layers.

By virtue of the positioning of the doped layer 16 and the control ofout-diffusion into the region 17, a semiconductor light emitting deviceor laser is formed which is highly efficient on the one hand and alsocapable of high power operation on the other. The buried stripe 18 isencompassed entirely within the semiconductor and is surrounded byregions of lower refractive index. More particularly, the first andsecond cladding layers which sandwich the active layer have lowerrefractive indices than the active layer and the portions of the activelayer bracketing the striped region 18 also have higher indices ofrefraction whereby the stripe region 18 in the active layer functions asa resonator to guide light generated in the stripe 18 toward the facets20, 20'. It is recalled that the stripe 16 has interposed portions 21,21' between the facets 20, 20' and the stripe. During the out-diffusionprocess, those gaps form windows 22, 22' (see FIGS. 1(a) and 1(c)) inthe active layer 13 intermediate the buried stripe 18 and the facets 20,20'. Because the windows 22, 22' are either undoped or n-doped AlGaAs,which has a relatively large band gap, whereas the doped stripe 18 has arelatively high (e.g., 10¹⁸ cm⁻³) concentration of p-type impurities,and thereby has a comparatively low band gap, substantially all of thelight produced within the buried stripe 18 will pass through the windows22, 22' without suffering the recombination typical of laser deviceshaving no window structure. More particularly, in prior art laserdevices without window structures, there exists a substantial number ofnon-radiative recombination centers near the facets which can absorbphotons which would otherwise be emitted, thereby creating additionalheat, lowering the energy band gap near the facets, exacerbating therecombination problem and ultimately destroying the device. By virtue ofthe window structure 22, 22' which has a substantially higher energy gapthan that of the stripe 18, the recombination problem is substantiallyeliminated and the device can be operated at high power for long periodsof time without risk of failure.

While in the above illustrated embodiment a GaAs/AlGaAs seriessemiconductor light emitting device is described, it will be appreciatedthat the present invention may be applied to a device or method ofproducing a semiconductor light emitting device of any series materialsuch as InP/InGaAsP series material.

It will now be apparent that what has been provided is a high powerlight emitting device such as a laser which is capable of being producedat high yield. A buried stripe in the active region is produced byout-diffusion, but the out-diffusion source is deposited in the claddinglayer immediately adjacent the active layer into which it is to beout-diffused. Thus, not only are windows provided interposed between theburied stripe and the laser facets, but by virtue of depositing thedoped impurities immediately adjacent the active layer into which theyare to grow, the growth can be controlled so that impurities areout-diffused into the active layer but only to the boundary between theactive layer and the second cladding layer. Thus, since the position ofthe p-n junction between the buried stripe and the second cladding layeris reliably controlled, the yield is substantially greater than has beenpossible using prior art techniques in which a first diffusion must beaccomplished through comparatively thicker contact and cladding layersbefore out-diffusion into the active layer can begin.

What is claimed is:
 1. A method of producing a buried stripesemiconductor light emitting device in which the buried stripe functionsas an internal resonator, and window regions are interposed between endsurfaces of the resonator and facets on the external surface of thelight emitting device, the method comprising the steps of:conducting afirst phase crystal growth in which a first cladding layer is grown on adoped substrate; introducing a doped stripe of impurities into the firstcladding layer in electrical contact with the doped substrate, the dopedstripe extending longitudinally but terminating short of the facet forforming said window regions; conducting a second phase crystal growth tobury the doped stripe internal of the semiconductor light emittingdevice, the second phase crystal growth including growing an activelayer, a second cladding layer and a contact layer successively over thefirst cladding layer; and out-diffusing impurities from the doped stripeinto the active layer to the boundary between the active layer and thesecond cladding layer to form the resonator, the out-diffused impuritiesforming an internal semiconductor junction between the resonator andsecond cladding layer having undoped window regions in the active layerintermediate the resonator and the facets.
 2. A method of producing aburied stripe semiconductor light emitting device as defined in claim 1wherein said doped stripe of impurities is introduced by diffusion.
 3. Amethod of producing a buried stripe semiconductor light emitting deviceas defined in claim 1 wherein said doped stripe of impurities isintroduced by ion injection.
 4. A method of producing a buried stripesemiconductor light emitting device as defined in claim 1 wherein thestep of conducting the second phase crystal growth is performed at thesame time as the step of out-diffusing impurities from said dopedstripe.
 5. A method of producing a buried striped semiconductor lightemitting device as defined in claim 4 wherein the step of conducting thesecond phase crystal growth is accomplished at an average temperature ofabout 700° C. for approximately 30 minutes.
 6. A method of producing aburied stripe semiconductor light emitting device as defined in claim 1wherein said semiconductor substrate and said doped stripe of impuritiesare of a first conductivity type, and said first cladding layer, saidsecond cladding layer and said contact layer are of a secondconductivity type, whereby a p-n junction is formed between theresonator and the second cladding layer.
 7. A method of producing aburied stripe semiconductor light emitting device as defined in claim 6wherein said active layer is grown from a second conductivity typematerial.
 8. A method of producing a buried stripe semiconductor lightemitting device as defined in claim 6 wherein said active layer is grownfrom an undoped material.
 9. A method of producing a buried stripesemiconductor light emitting device as defined in claim 1 wherein Znimpurities are diffused into the first cladding layer to form the dopedstripe, the doped stripe has an impurity concentration of approximately10²⁰ cm⁻³, and the out-diffused impurity concentration in the resonatoryis approximately 10¹⁸ cm⁻³.